1. Field of the Invention
The present invention relates to a method for picking up dies from a semiconductor wafer on which a plurality of dies are formed.
2. Prior Art
Generally, wafers have a round shape, and a plurality of dies which have a square shape are formed in longitudinal and lateral rows on such wafers. As a result, the edges of dies located in the peripheral area of the round wafer have a circular arc shape, and the prescribed die shape cannot be obtained in such dies. Thus, such dies are called defectively shaped dies. Furthermore, the dies near defectively shaped dies often do not have completely formed circuits. Such dies on which no complete circuits are formed are generally called "mirror dies", and these mirror dies are defective. Recently, therefore, there has been a tendency of dies deliberately not formed in the peripheral areas of the round wafers.
Conventionally known methods detect dies having a defective shape in the peripheral areas of a wafer are disclosed in, for example, Japanese Patent Application Laid-Open (Kokai) No. 56-98835.
In the method disclosed in this prior art, the external shapes of the dies are detected by a detecting device that is moved in a detecting direction; and when a defectively shaped die is detected, the detecting device is moved to the next row and reverses the direction of detection (i. e., the direction in which die pick-up proceeds) and continues the detection.
Another method is disclosed in Japanese Patent Application Publication (Kokoku) No. 1-45221. In this method, the detecting device is moved to the next row and reverses the direction of detection when no die has been detected several consecutive times from the circumference of the wafer.
The prior arts described above, however, in fact, do not detect mirror dies or dies resembling mirror dies (which contain defective circuits) in the peripheral areas of the wafer. Accordingly, when these techniques are used to detect mirror dies or dies resembling mirror dies, the number of times to detect dies increases, and there is a significant drop in productivity.